1. Field of the Invention
The present invention relates to a battery and, more particularly, to a battery in which a plurality of cells are linked so as to obtain a required power capacity.
2. Description of the Related Art
Large-capacity batteries comprising a plurality of linked cells, such as nickel-cadmium cells, nickel-hydrogen cells, or hydrogen cells, are used in various electric appliances, electric vehicles, and the like. In such a large-capacity battery, typically, a plurality of cells in the shape of a thin rectangular parallelepiped are arranged close to each other and bound together. In each cell, a plurality of positive electrode plates and a plurality of negative electrode plates are alternately laminated via separators containing electrolytic solution. For such a cell, when the ambient temperature is high or a large amount of current is discharged, heat is not sufficiently dissipated from the electrode plates contained in each cell, causing the temperature of the cell to be increased, potentially leading to a reduction in the life of the battery. In order to avoid such a problem, configurations described below have been proposed for cooling a large-capacity battery comprising a plurality of linked cells.
Japanese Laid-Open Publication No. 2000-164186 discloses a battery. In the battery, a plurality of cells (each cell being in the shape of a rectangular parallelepiped having a width direction dimension greater than a thickness direction dimension) are connected in series so that the sides along the width direction (width direction side) of the cells are located on the same plane. On the width direction side of each cell, a plurality of ribs are provided in a vertical direction. A coolant channel, through which a coolant is forced to flow, is provided between each rib in the vertical direction of the cell. Thereby, each cell is cooled.
Japanese Laid-Open Publication No. 6-215804 discloses a monoblock battery. The battery is in the shape of a rectangular parallelepiped. A side plate is provided along each of the wall surfaces in the width direction of the battery. A coolant channel (fluid circulation space) is provided between the wall surface and the side plate, and a coolant is supplied to the coolant channel.
Japanese Laid-Open Publication No. 2000-251950 discloses another battery. In the battery, a plurality of cells are linked and arranged so that the width direction sides of the cells face each other. A coolant channel is provided between each cell. Another coolant channel is provided on the sides in the thickness direction of the cells for allowing the coolant channels to communicate with each other. A coolant is allowed to flow through the coolant channel.
However, the configuration disclosed in Japanese Laid-Open Publication No. 2000-164186 described above requires an additional structure for distributing the coolant to the coolant channels of the cells. For this reason, the configuration of the entire battery is complicated and therefore the number of assembling steps is increased, causing an increase in cost.
In the configuration of Japanese Laid-Open Publication No. 6-215804 described above, only the sides of the battery comprising a plurality of linked cells are cooled. Therefore, when a great load is applied to the battery and therefore the amount of heat generated is great, it is difficult to obtain a sufficient cooling effect. Therefore, in order to obtain a sufficient cooling effect, the battery requires an additional structure for distributing the coolant to the coolant channels of the cells. As in Japanese Laid-Open Publication No. 2000-164186, the configuration of the entire battery is complicated and therefore the number of assembling steps is increased, causing an increase in cost.
In the configuration of Japanese Laid-Open Publication No. 2000-251950 described above, the coolant is allowed to flow mainly in the thickness direction side of each cell, and the amount of the coolant flowing through the coolant channel provided between each cell is small. In each cell, a plurality of electrode plates are alternately laminated, and the electrode plates are arranged along the width direction side of the cell. In addition, space is provided between the thickness direction side and the electrode plate so as to facilitate production of the battery. In order to obtain higher cooling efficiency, it is necessary to allow the coolant to flow along the width direction side. However, in this configuration, the coolant is allowed to flow mainly along the thickness direction side and therefore a sufficient level of cooling efficiency cannot be obtained.
As described in this publication, a plurality of cells are arranged so that the width direction sides face each other, and are integrally bound. The cells provided on the ends of the battery receive a smaller level of pressure. In such cells, therefore, electrolytic solution is likely to be dried up, so that the life of the cells is significantly smaller than that of the other cells. This situation will be specifically described below.
FIG. 40 is a schematic diagram showing a configuration of a conventional battery. FIG. 41 is a diagram for explaining expansion of the cell in the conventional battery. Referring to FIG. 40, the conventional battery 400 comprises 6 cells 401, 402, 403, 404, 405, and 406, each of which is in the shape of a rectangular parallelepiped in which the width direction dimension is greater than the thickness and height direction dimensions. The cells are arranged so that the width direction sides thereof face each other, and are integrally bound. In each cell, a plurality of electrode plates (positive electrode plates and negative electrode plates) are laminated, and arranged along the width direction sides of the cells. In the battery 400 having the above-described configuration, when discharging cycles are repeated in each cell 401 to 406, each electrode plate expands. Therefore, as shown in FIG. 41, the cells 401 to 406 expand in a direction away from the cells 403 and 404 provided at the center, toward the outside. In this case, for the cells 401 to 406, the further out the location of the cell, the smaller the binding force applied to the cell, therefore, the greater the expansion of the cell.
When the cells 401 to 406 expand in this manner, the outer cells expand to a greater extent. Therefore, the further out the cell, the smaller the pressure acting on the electrode plate. If the pressure acting on the electrode plate becomes small, the distance between adjacent electrode plates becomes great, causing the electrolytic solution to splash so that the electrolytic solution is likely to be dried up.
FIG. 42 is a graph showing the life characteristics of cells in a conventional battery. As described above, the further out the cell, the smaller the pressure applied to the cell. Therefore, the pressure acting on the cells 401 and 406 located on the ends of the battery are small as compared to that acting on the other cells 402 to 405, whereby the electrolytic solution is likely to be dried up. Consequently, as shown in FIG. 42, the life of the cells 401 and 406 on the ends of the battery is significantly smaller than the life of the other cells 402 to 405, causing variations in the life of the cells in the battery.
In order to suppress the expansion of the cells 401 to 406, a configuration has been proposed, in which as shown in FIG. 43, expansion suppressing plates 411 and 412 are provided on the respective ends of the battery. The expansion suppressing plates 411 and 412 integrally bind all of the cells 401 to 406. Despite this configuration, the expansion of the cells 401 and 406 provided on the ends of the battery cannot be sufficiently suppressed.
FIG. 44 is a graph showing a temperature distribution of the cells when the battery shown in FIG. 40 is employed in an EV (electric vehicle). When the battery of FIG. 40 is applied to an EV in which a large amount of current may be input or output, variations in temperature between each cell 401 to 406 are large as shown in FIG. 44. Points 401B to 406B indicate the temperatures of the cells 401 to 406, respectively. The temperatures of the cells 403 and 404 provided at the middle of the battery are high. The closer the location of the cell to the opposite ends of the battery, the lower the temperature of the cell. Thus, the variations in temperature between the cells 401 to 406 are large, and the temperatures of the cells provided in the middle are higher. In this case, corrosion of the grid-like electrode plate and degradation of active substances provided in the electrode plate are accelerated, causing an early reduction in the output voltage of the cell, so that the life of the battery is reduced.
In each of the above-described conventional batteries, a liquid coolant, such as water, is used as a coolant for cooling the linked cells at predetermined positions. Power generation elements composed of positive electrode plates, negative electrode plates and separators are completely shielded from the coolant channels in order to prevent the liquid coolant from penetrating into the power generation elements.
For example, in Japanese Laid-Open Publication No. 6-215804 described above, a plastic material case comprising a bath having an open top, which contains power generation elements, such as electrode plates, and a lid attached to a top portion of the bath, seals the power generation elements so that the power generation elements are shielded from the coolant channel.
In Japanese Laid-Open Publication No. 2000-251950 described above, a plurality of cells are integrally linked in series to construct a sealed secondary battery. A lid member is attached to a top portion of the sealed secondary battery, whereby power generation elements in the cells are sealed, and shielded from a cooling channel.
However, in these publications, the lid is attached to the bath containing the cells, although positioning means for attaching the lid to an appropriate position is not provided. Therefore, it is not easy to appropriately position the lid with respect to the bath. If the lid is not correctly positioned with respect to the bath, the cells are not effectively cooled by the coolant. Also, when the battery is used in a situation where wobble or the like may occur, the lid is displaced from the bath, whereby the coolant is likely to penetrate into the power generation element.
The portion of the battery, which generates heat, is not limited to the electrode plate. In particular, when a terminal portion, which is externally connected, excessively generates heat, a portion around the terminal of the bath containing the cells may be melted. However, in the above-described publications, the electrode plate is mainly cooled by the coolant, but the battery is not provided with an arrangement for preventing the heat generation of the terminal portion.
Next, a problem with the internal structure of conventional batteries having a configuration in which positive electrode plates and negative electrode plates are laminated via separators will be described below.
FIG. 45 is a perspective view showing an exemplary internal structure of a conventional battery 1.
A battery 501 has a case body 502 which is in the shape of a hollow rectangular parallelepiped and has an open top. The internal space of the case body 502 is divided by a partition 503 into three in a longitudinal direction and two in a width direction, i.e., 6 cells 502a to 502f. The cells 502a to 502f each have a cross section in the shape of a rectangle extending in the longitudinal direction of the case body 2.
The cells 502a to 502f each contain a unit power generation element having a plurality of positive electrode plates (e.g., PbO2 plate), each of which has a similar planar shape, and a plurality of negative electrode plates (e.g., Pb plate), each of which also has a similar planar shape. In the unit power generation element, positive electrode plates and negative electrode plates are alternately laminated via separators made of porous, extremely fine glass fibers holding dilute sulfuric acid, or the like.
At one end of the case body 502, the first cell 502a and the sixth cell 502f are disposed side by side in the width direction of the case body 502. The first cell 502a, the second cell 502b and the third cell 502a are disposed side by side in a longitudinal direction of the case body 502. The third cell 502a and the fourth cell 502d are disposed side by side in the width direction of the case body 502. The fifth cell 502e is disposed between the fourth cell 502d and the sixth cell 502f. The positive electrode plates and the negative electrode plates of the unit power generation element of each cell 502a to 502f each extend in the longitudinal direction of the case body 502.
All of the positive electrode plates of each unit power generation element of the second cell 502b to the fifth cell 502e (i.e., excluding the first cell 502a and the sixth cell 502f) are connected to a first strap 504 provided on one side of the positive electrode plate. All of the negative electrode plates of each unit power generation element are connected to a second strap 504 provided on a side of the negative electrode plate opposite to the first strap 504 provided on the side of the positive electrode plate. The first strap 504 is conductive to all of the positive electrode plates, while the second strap 504 is conductive to all of the negative electrode plates.
All of the positive electrode plates of the unit power generation element contained in the first cell 502a are connected to the strap 504, while all of the negative electrode plates are connected to a terminal member 505. All of the negative electrode plates of the unit power generation element contained in the sixth cell 502f are connected to the strap 504, while all of the positive electrode plates are connected to the terminal member 505.
The strap 504 connected to the negative electrode plate of the unit power generation element contained in the first cell 502a, is interconnected to the strap 504 which is connected to the positive electrode plates of unit power generation element contained in the second cell 502b, via a through hole provided in the partition 503. As shown in FIG. 45, the strap 504 connected to the negative electrode plates of the unit power generation element in the second cell 502b, is interconnected to the strap 504 connected to the positive electrode plates of the unit power generation element in the third cell 502c, via a through hole provided in the partition 503. The strap 504 connected to the negative electrode plates of the unit power generation element in the third cell 502c, is interconnected to the strap 504 connected to the positive electrode plates of the unit power generation element in the fourth cell 502d, next to the third cell 502c in the width direction of the case body 502, via a through hole provided in the partition 503.
The strap 504 connected to the negative electrode plates of the unit power generation element in the fourth cell 502d, is interconnected to the strap 504 connected to the positive electrode plates of the unit power generation element in the fifth cell 502e, via a through hole provided in the partition 503. The strap 504 connected to the negative electrode plates of the unit power generation element in the fifth cell 502e, is interconnected to the strap 504 of the positive electrode plates of the unit power generation element in the sixth cell 502f, via a through hole provided in the partition 503. Thus, the unit power generation elements contained in the cells 502a to 502f are connected in series. The terminal member 505 connected to the unit power generation element in the first cell 502a is a positive terminal, while the terminal member 505 connected to the unit power generation element in the sixth cell 502f is a negative terminal.
FIG. 45 is a front view of an electrode plate 510 constituting the positive electrode plate or the negative electrode plate contained in the cells 502a to 502f of the conventional battery 501. The electrode plate 510 has a rectangular electrode plate body 513 and a rectangular collector 511 which is provided at a side of the electrode plate body 513, and projects from the electrode plate body 513 upward. The collector 511 is provided at the side edge of the electrode plate body 513, leaving an appropriate spacing with respect to an end of the side of the electrode plate body 513, and also leaving an appropriate spacing with respect to the center of the side of the electrode plate body 513.
The thus-constructed electrode plate 510 is used in a manner as shown in FIG. 46A. Specifically, a pair of the electrode plates 510 are attached together via a separator, where the collectors 511 are positioned on the opposite sides, i.e., one of the electrode plates 510 is turned from side to side (by 180°) and is then attached to the other electrode plate 510 to obtain a positive electrode plate and a negative electrode plate.
In the unit power generation elements contained in the second cell 502b to the fifth cell 502e, as shown in FIG. 46B, one strap 504 is connected by welding to the collectors 511 of all of the electrode plates 510 constituting the positive electrode plates, while the other strap 504 is connected to the collectors 511 of all of the electrode plates 510 constituting the negative electrode plates.
As shown in FIG. 47, the strap 504 has an electrode plate connector 504a, which is in the shape of a plate and is attached by welding to a top edge of the collector 511 provided in the electrode plate 510, and an inter-cell connector 504b which is bent extending upward from a side of the electrode plate connector 504a. The electrode plate connector 504a is attached by welding to the collector 511 of the electrode plate 510 constituting a positive electrode plate or a negative electrode plate, where the inter-cell connector 504b is disposed along the partition 503 provided between the adjacent cells.
The collector 511 provided in the electrode plate 510 is made of the same material as that of the electrode plate 510 (e.g., lead (Pb) or lead oxide (PbO2)). Therefore, the collector 511 has a considerably large weight. It is preferable to reduce the width direction length of the collector 511 in order to reduce the weight of the collector 511.
However, the strap 504 provided on the top portion of the collector 511 has to have a width direction length greater than the width direction length of the collector 511. If the width direction length of the collector 511 is excessively smaller than the width direction length of the strap 504, damage, such as rupture, may occur around the collector 511 due to wobble or the like. Therefore, an appropriate ratio of the width direction length of the strap 504, to the width direction length of the collector 511, is important in order to avoid damage, such as rupture, and to reduce the weight of the battery.
When the width direction length of the strap 504 provided on the top end of the collector 511 is excessively small as compared to the width direction length of the electrode plate 510, the resistance of the strap 504 is high and a voltage drop is large in the case of discharging a large amount of current. Therefore, an appropriate ratio of the width direction length of the strap 504 to the width direction length of the electrode plate 510 is important in order to prevent a voltage drop.